There are many reasons why they couldn’t always identify bodies.

The most obvious one is that sometimes the body was just too damaged. There were many cases of living men with horrific injuries - if you look up a man called Harold Gillies, you should see photographs of some of his patients, men whose faces had essentially been blown off by shrapnel. If that’s what the living could look like, imagine what might have happened to the dead. How do you identify an individual person in a jigsaw puzzle pile of limbs and torsos, when you’re still decades away from DNA identification? Identifying damaged or decomposed bodies is still time consuming and difficult now! With no DNA, and much more basic medical records to refer back to, it would take forever.

Personal identification like “dog tags” were officially required, but it had been a relatively recent development - Britain first introduced a tag system in 1907. These early tags were a flawed system too; easily lost and easily damaged compared to the modern version, and without the benefit of a database that could be searched quickly as we have now.

Confusion among men with similar names (imagine how many Private Evanses there must have been among the Royal Welsh Fusilliers) wouldn’t have helped either.

Almost all of those missing men with their names on the Menin Gate will have a grave somewhere. It just won’t be under their own name - they’ll be one of the many buried as “Known Unto God”. The gate is the only place where their names can be.

Heavy shelling also had a very real effect on the landscape. You were in Ypres. Look at before and after pictures of the Third Battle of Ypres, Passchendaele. In the after shots, it looks like the surface of the moon. Drowning in the mud was a very real, horrific possibility as a way to die. There may be records of losing fifty men, but when ten of them are buried under tons of mud that fell on them when their trench collapsed from being shelled...you can’t get those men out, can you? You can’t look to see which ten men they are or if their tags are still there. They just have to stay there, under mud as heavy as a house. Farmers in Belgium still turn up human remains with their tractor each year as they do the ploughing.

Last of all, there was the simple fact that some of these men had lied. False names, lying about their age (the youngest soldier killed that I can recall right now was a guy called James Charles Martin, and when he died he was barely fourteen)...they weren’t always being honest with what they told the army in the first place, which complicates any identification efforts later.